Permanent magnets find applications in a variety of devices, including electric motors for hybrid and electric vehicles. Sintered Nd—Fe—B permanent magnets have very good magnetic properties at low temperatures. After magnetization, permanent magnets are in a thermodynamically non-equilibrium state. Any changes in the external conditions, in particular the temperature, result in a transition to another, more stable state. These transitions are typically accompanied by changes in the magnetic properties. Due to the low Curie temperature of the Nd2Fe14B phase, the magnetic remanence and intrinsic coercivity decrease rapidly with increased temperature.
It is important to improve the thermal stability of this material and to increase the magnetic properties further in order to obtain compact, lightweight, and powerful motors for hybrid and electrical vehicles. There are two common approaches to improving thermal stability and magnetic properties. One is to raise the Curie temperature by adding Co, which is completely soluble in the Nd2Fe14B phase. However, the coercivity of the Nd—Fe—B magnets with Co decreases, possibly because of the nucleation sites for reverse domains. The second approach is to add heavy rare-earth (RE) elements. It is known that the substitution of dysprosium for neodymium or iron in Nd—Fe—B magnets results in increases of the anisotropic field and the intrinsic coercivity, and a decrease of the saturation magnetization (C. S. Herget, Metal, Poed. Rep. V. 42, P. 438 (1987); W. Rodewald, J. Less-Common Met., V111, P 77 (1985); and D. Plusa, J. J. Wystocki, Less-Common Met. V. 133, P. 231 (1987)). It is a common practice to add the heavy RE metals such as dysprosium (Dy) or terbium (Tb) into the mixed metals before melting and alloying.
However, Dy and Tb are very rare and expensive RE elements. Heavy REs contain only about 2-7% Dy. The price of Dy has increased sharply in recent times (from about $50/kg for DyO in 2005 to about $140/kg in 2010). Tb is needed if higher magnetic properties are required than Dy can provide, and it is much more expensive than Dy (about $400/kg for TbO).
Typical magnets for motors in hybrid vehicles contain about 6-10 wt. % Dy to meet the required magnetic properties. Conventional methods of making magnets with Dy or Tb result in the Dy or Tb being uniformly distributed within the magnet.
Assuming the weight of permanent magnet pieces is about 1-1.5 kg per electric motor, and a yield of the machined permanent magnet (PM) pieces of typically about 55-60%, 2-3 kg of PM per motor would be required, or 4-6 kg per vehicle (some hybrid vehicles may use one induction motor and one PM motor). Moreover, Dy is also widely used by other industries. The only RE mine in the United States does not have any significant amounts of Dy. Therefore, reducing the Dy or Tb usage in permanent magnets would have a very significant cost impact.
Nd—Fe—B permanent magnets can be produced using a powder metallurgy process, which involves melting and strip casting, hydrogen decrepitation (hydride and de-hydride), pulverizing (with nitrogen), screening, and mixing alloy powders for the desired chemical composition. A typical powder metallurgy process follows: weighing and pressing (vacuum bagging), isostatic pressing, sintering and aging (e.g., about 30 hrs, at about 1100 C, in vacuum), and machining to magnet pieces. Finally, the magnets are surface treated by phosphating, electroless Ni plating, epoxy coating, etc.
The ideal microstructure for sintered Nd—Fe—B based magnets is Fe14Nd2B grains perfectly isolated by the nonferromagnetic Nd-rich phase (a eutectic matrix of mainly Nd plus some Fe4Nd1.1B4 and Fe—Nd phases stabilized by impurities). The addition of Dy or Tb leads to the formation of quite different ternary intergranular phases based on Fe, Nd and Dy or Tb. These phases are located in the grain boundary region and at the surface of the Fe14Nd2B grains. The addition of elements to improve the magnetic properties should desirably fulfill the following conditions: 1) the intermetallic phase should be nonferromagnetic to separate the ferromagnetic grains; 2) the intermetallic phase should have a lower melting point than the Fe14Nd2B phase to produce a dense material via liquid phase sintering; and 3) the elements should have a low solubility in Nd2Fe14B to keep good magnetic properties.
The microstructures of Nd—Fe—B sintered magnets have been extensively investigated in order to improve the magnetic properties. In general, sintered magnets are mainly composed of the hard-magnetic Nd2Fe14B phase and a nonmagnetic Nd-rich phase. The coercivity is known to be greatly influenced by the morphology of the boundary phases between Nd2Fe14B grains. The magnetic properties of the Nd—Fe—B sintered magnets are degraded when the magnet size is decreased because the machined surface causes nucleation of magnetic reversed domains. Machida et al. (Machida, K., Suzuki, S., Ishigaki, N., et al., Improved magnetic properties of small-sized magnets and their application for DC brush-less micro-motors. Coll. Abstr. Magn. Soc. Jpn. 142 (2005), 25-30), found that the degraded coercivity of small-sized Nd—Fe—B sintered magnets can be improved by surface treating of the formed magnet with Dy and Tb-metal vapor sorption so that there is a uniformly distributed coating of Dy or Tb on the outside of the formed magnet and no Dy or Tb inside.